A purpose of direct discovery is to enable one device to discover neighbor devices by receiving discovery signals broadcasted from these neighbor devices directly. A commercial example of direct discovery is a commercial store broadcasting advertisements to customers by delivering discovery signals. Generally, the entity which broadcasts discovery signals is called Tx DUE (Transmitter Discovery User Equipment) and the entity which receives discovery signals is called Rx DUE (Receiver Discovery User Equipment). A device may be a Tx DUE, an Rx DUE, or it may also play both of the roles. Because the purpose of direct discovery is to enable user to obtain the local information around the DUE, Tx DUE would radiate low transmission power to deliver discovery signals. In some of the implementation, Tx DUE may deliver discovery signal with ˜20 dBm radiation power.
Take the direct discovery service based on the 3GPP's agreement as an example. A so-called master base station (MBS), or an extended Node B (eNB) in 3GPP specifications, for example, could be a macro base station, or a master micro base station, etc. As shown in FIG. 1, a macro BS provides radio resource for in-coverage DUE to implement direct discovery service and configures radio resource for in-coverage devices to broadcast discovery signals.
The configuration may be a pre-stored list in the memory device or USIM (Universal Subscriber Identity Module) of the DUE. Currently, there are two resource allocation approaches, contention-free and contention-based resource allocations, that are provided to discovery service. The contention-based resource allocation may assist to both MBS and devices: 1) Because of low radiation power used in direct discovery, it is possible that different Tx DUEs may share the same resource blocks to deliver their discovery signals. 2) Contention-based resource allocation enables Tx DUE to stay in the idle state, which could save the battery life time and hardware computations of Tx DUE. 3) Cellular network does not need to keep the UE information about those Tx DUEs, which would reduce the burden of cellular networks, especially when the number of Tx DUEs is large.
As shown in FIG. 1, a sub-frame may be the minimum time unit for MBS to decide whether to allocate D2D (device-to-device) radio resource(s) or not in each sub-frame. DUE may also be configured with a list of direct discovery frequencies which provides direct discovery radio resource pool. MBS may configure radio resources of one or more than one component carriers. In each component carrier, the MBS could allocate communication radio resource pools, e.g., discovery radio resource pool, scheduling assignment message pool, data resource pool, etc., in each sub-frame dynamically or semi-persistently. The resource pool is a block of radio resource which is a time period in the time domain and one or more than one frequency subcarriers in the frequency domain.
After receiving the configuration of direct discovery radio resource pool from the MBS, Tx DUEs may transmit discovery signals on a randomly selected resource block, which is a subset of discovery radio resource pool. Rx DUE would also monitor these configured radio resource pool to obtain discovery signals. The tradeoff between power consumption and processing time of radio resource management of the DUE is often a design consideration.